Re: [RFC PATCH 0/7] Introduce thermal pressure

[Thara Gopinath <thara.gopinath@linaro.org>]

On 10/16/2018 03:33 AM, Ingo Molnar wrote:
> 
> * Thara Gopinath <thara.gopinath@linaro.org> wrote:
> 
>>>> Regarding testing, basic build, boot and sanity testing have been
>>>> performed on hikey960 mainline kernel with debian file system.
>>>> Further aobench (An occlusion renderer for benchmarking realworld
>>>> floating point performance) showed the following results on hikey960
>>>> with debain.
>>>>
>>>>                                         Result          Standard        Standard
>>>>                                         (Time secs)     Error           Deviation
>>>> Hikey 960 - no thermal pressure applied 138.67          6.52            11.52%
>>>> Hikey 960 -  thermal pressure applied   122.37          5.78            11.57%
>>>
>>> Wow, +13% speedup, impressive! We definitely want this outcome.
>>>
>>> I'm wondering what happens if we do not track and decay the thermal 
>>> load at all at the PELT level, but instantaneously decrease/increase 
>>> effective CPU capacity in reaction to thermal events we receive from 
>>> the CPU.
>>
>> The problem with instantaneous update is that sometimes thermal events 
>> happen at a much faster pace than cpu_capacity is updated in the 
>> scheduler. This means that at the moment when scheduler uses the 
>> value, it might not be correct anymore.
> 
> Let me offer a different interpretation: if we average throttling events 
> then we create a 'smooth' average of 'true CPU capacity' that doesn't 
> fluctuate much. This allows more stable yet asymmetric task placement if 
> the thermal characteristics of the different cores is different 
> (asymmetric). This, compared to instantaneous updates, would reduce 
> unnecessary task migrations between cores.
> 
> Is that accurate?

Yes. I think it is accurate. I will also add that if we don't average
throttling events, we will miss the events that occur in between load
balancing(LB) period.

> 
> If the thermal characteristics of the cores is roughly symmetric and the 
> measured CPU-intense load itself is symmetric as well, then I have 
> trouble seeing why reacting to thermal events should make any difference 
> at all.
In this scenario, i agree that scheduler reaction to thermal events
should not make any difference in fact we should not observe any
improvement or degradation in performance.

> 
> Are there any inherent asymmetries in the thermal properties of the 
> cores, or in the benchmarked workload itself?

The benchmarked workload , meaning aobench? I don't think there arre any
asymmetries there. On Hikey960, there are two clusters with different
frequency domains. So yes I will say there is asymmetry there. Asides,
IMHO, any other tasks running on the system can create an inherent
asymmetry as cpu utilizations can vary.

Regards
Thara
> 
> Thanks,
> 
> 	Ingo
> 


-- 
Regards
Thara